While this isn't for everyone, I assumed from reading your post that you  would be a candidate for an attorney w/ that manages Trust accounts (Accounts established on client's behalf & managed locally by the attorney.)  

The toughest part for you will be establishing the 2 years of stable income for the underwriter considering so much of a personal trainers income is variable.

In most jurisdictions, once a person has established residency, they will be considered your tenants and will have equal tenant rights of tenancy, whether or not their name is on the rental agreement and whether or not they were the original tenant.

Once I got that established or all the people in the field moved over, I'd look at putting in another strip that has a gravel base and is covered.Depending on your location and what your customers want depends on the next step. 

Which would hit the 10 unit limit in accordance with the General Plan.RM-4300 Residential, Multi-family, 1 unit/4,300 square feet of total lot area or, 44,000 sq/ft lot / 4,300 = ~10 unitsThen finance the construction on a 10 unit complex.Keeping the property as a long-term rental complex seems like an easier process than splitting the lot up, establishing an HOA and selling individual lots.

having an established rental portfolio and strong cash flow to back those risks.

I agree with @FredHeller on what he wrote about establishing cost of any property you purchase.

I plan on establishing low-income housing for these apartments.

So it's not a cure all for saving out of pocket costs, else the rates would be a lot higher. 

Can I just establish a small business (sole proprietorship) and file along with my personal taxes and social security number?